type Point = { x: number, y: number };

type P = keyof Point;
// 等同于 'x' | 'y'

type Arrayish = { [n: number]: unknown };
// type A = number
type A = keyof Arrayish;


type Mapish = { [k: string]: boolean };
// type B = string | number
type B = keyof Mapish;

export {};